This invention relates to techniques for increasing the density of recorded digital information on magneto-optical storage media.
The exploding demand for computer memory has propelled current research in memory systems in the direction of magnetooptical (M-0) technology. The M-0 medium, typically in the form of a disk, comprises a thin magnetic layer covered with a relatively thick transparent coating. Digital information is stored in the M-0 medium by locally magnetized regions or "domains" in the magnetic layer of one polarity or another corresponding to 1's" and "0's". While the information is thus retained magnetically in a manner analogous to conventional magnetic media, the writing and reading processes usually involve laser beams. M-0 writing is thermally assisted. A pulsed laser beam is focused through the transparent overcoat onto the surface of the magnetic layer. The coercivity of the magnetic media exposed to the beam is temporarily lowered by the heat induced by the laser pulse, enabling the local orientation of the magnetic domains to be redirected by means of a magnetic field. Reading is accomplished through Kerr or Faraday rotation of the angle of polarization of a low power (non-heating) incident laser beam (ordinarily supplied by the same laser used in writing). Depending on the local orientation of the magnetic media, the polarization angle of the reflected beam rotates slightly clockwise or counterclockwise. This shift in the polarization angle determines whether the cell contains a "1" or a "0".
A known magneto-optical drive system is shown in FIG. 1. The diskette 10 is written and read by means of a focused laser beam 12. The beam 13 is focused by lens 14. As shown in FIG. 2, disk 10 contains, in a radial recording region 16 of one face 15, a spiral track of spaced magnetic domains. Alternatively, the recording region 16 can carry plural concentric circular tracks. As shown in FIG. 3, the track width 18 must be equal to or greater than the width of the intense portions of the focused laser beam 17 to avoid interference between domains in adjacent turns of the spiral magnetic track.